Aniline is a colorless, oily liquid with a strong odor and toxicity. It is an important chemical intermediate, and thus is widely used for producing rubber processing aids, dyes, photographic chemicals, pharmaceuticals, pesticides, explosives, polyurethanes and the like. The production methods of aniline comprise phenol ammoniation, iron powder reduction and catalytic hydrogenation of nitrobenzene.
Taking into account the source of raw materials, energy consumption, environmental protection and so on, most manufacturers use catalytic hydrogenation of nitrobenzene. The catalytic hydrogenation may be divided into two processes, i.e., the fixed bed gas phase hydrogenation and fluidized bed gas phase hydrogenation according to the different forms of the reactor used. The fixed bed gas phase hydrogenation has the advantages of simple process, low maintenance cost, no need for separation of catalyst, low reaction temperature, good product quality and the like, but local overheating (hot spot) is prone to occur due to poor heat transfer of the fixed bed, which leads to side reactions and deactivation of the catalyst. Therefore, the activity cycle of the catalyst is shortened. For the fluidized bed reactor, heat transfer is good, such that local overheating may be avoided, side reactions may be reduced and the service life of the catalyst may be prolonged. The fluidized bed reactor has the disadvantages of complicated operation, remarkable catalyst abrasion, and high operation and maintenance costs. The liquid phase hydrogenation comprises a gas phase reaction and a liquid phase reaction and achieves easy reaction heat removal, and has the advantages of simple equipment and low operation and maintenance costs, but the cost of technology introduction is high.
Usually, aniline is produced by the liquid phase catalytic hydrogenation process, wherein the raw material nitrobenzene is desulfurized and decarbonized by an oxidation system, the nitrobenzene and hydrogen gas are mixed and pretreated, and then reacted in a fluidized bed reactor and the used catalyst is recycled to the reactor for reuse. Since water is generated in the reaction, the crude product obtained after reaction is refined by extraction, the unreacted nitrobenzene is recovered, aniline is obtained by rectification, and aniline rectification residue is enriched in the rectification tower kettle. The aniline rectification residue is primarily originated from the two aspects: (1) a part of high boiling components is produced from nitrobenzene due to the excess of hydrogen gas in the hydrogenation; (2) cyclohexanone intermediate produced in the hydrogenation reaction may be reacted with aniline to form Schiff base tar, which is black viscous liquid with a pungent smell and contains a lot of aromatic substances with negatively charged π-electron system, such as
cyclohexyl aniline
diphenyl amine
1,2,3,4-tetrahydrocarbazole
p-(phenylamino) aniline
phenol, N-cyclohexyl-1,2-phenylenediamine
2-phenyl aniline
long chain alkanes and the like. These substances have higher molecular weight, and if the molecules contain an amino group (—NH2), two molecules containing such amino group will take place deamination reaction, so as to form a large molecule containing —NH—. These factors make aniline rectification residue have high viscosity and poor fluidity, and thus difficult to be reused. It is generally used for firewood material and waterproof material. The amount of aniline rectification residue used for the two uses is very small, based on the total amount of aniline rectification residue. Therefore, most of the aniline rectification residue, as waste liquid, is treated by incineration, but after incineration, it will produce nitrogen oxides and lead to acid rain and pollution environment, and further reduce the output of agriculture and fisheries.
The technical problem to be solved by the present invention is how to crack the aniline rectification residue sufficiently to obtain a certain amount of high economic value of product, in order to turn it from waste to treasure and reduce the pollution of the environment as far as possible. At present, there is no effective method for the utilization of aniline rectification residue.